Project Summary Working memory, the ability to temporarily hold multiple pieces of information in mind for manipulation, is central to virtually all cognitive abilities. Recent technical advances have opened an unprecedented opportunity to comprehensively dissect the neural circuit mechanisms of this ability across multiple brain areas. The task to be studied is a common form of decision-making that is based on the gradual accumulation of sensory evidence and thus relies on working memory. A team of leading experts propose to investigate the neural basis of this behavior using the latest techniques, including virtual reality, high-throughput automated behavioral training, large-scale cellular-resolution imaging in behaving rodents, manipulation of neural activity in specific brain areas and cell types, and automated anatomical reconstruction. In particular, the researchers will identify key brain regions that are required for this decision task through systematic, temporally specific inactivations via optogenetics technology, across all of dorsal cortex and in key subcortical areas, and use quantitative model-fitting to evaluate the effects. They will use state-of-the-art two-photon calcium imaging methods and electrophysiology to characterize the information flow in many individual neurons within these brain areas during the task. In addition, they will use cutting-edge anatomical reconstructions and new functional connectivity methods, within and across brain regions, to evaluate the interactions of these physiologically characterized neurons. The long-term goal of this project is to arrive at a complete, brain-wide understanding of the cellular and circuit mechanisms of activity dynamics related to working memory. Finally, they will use sophisticated computational methods to incorporate this new understanding into a realistic circuit model that will support a tightly integrated process of model-guided experimental design, in which the model suggests the most informative experiments and their results are then fed back to improve the model?s fidelity. This process is expected to produce the most accurate and detailed multi-brain-region biophysical circuit model of a cognitive process in existence. In addition, the proposed research will enable researchers to generate and test a variety of hypotheses about the neural basis of evidence accumulation, working memory, and decision-making. Taken together, these achievements will represent a crucial step toward a mechanistic understanding of how the brain works with information.